gpio: moxart: fix build regression

A cleanup patch replaced bgpio_chip with gpio_chip but missed
two references to the bgpio_chip:

drivers/gpio/gpio-moxart.c:60:19: error: use of undeclared identifier 'bgc'; did you mean 'gc'?
        gc->bgpio_data = bgc->read_reg(bgc->reg_set);
drivers/gpio/gpio-moxart.c:35:20: note: 'gc' declared here
drivers/gpio/gpio-moxart.c:60:33: error: use of undeclared identifier 'bgc'; did you mean 'gc'?
        gc->bgpio_data = bgc->read_reg(bgc->reg_set);

This adds the missing change.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 0f4630f3 ("gpio: generic: factor into gpio_chip struct")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>